I recently took on the challenge of adding a shower to my existing downstairs bathroom. The bathroom is inside what used to be the garage so it is far away from any existing vents. After some demo, and finding out that I do not need a permit for the project, the existing pluming looks like this (I hope you can figure out my diagram):

enter image description here

I am pretty sure that the 3x3x1.5 90 degree elbow under the toilet is not up to code. Also, in this configuration we had some issues with the toilet backing up a lot.

As my township uses International Code Council (ICC), I am pretty sure that both wet venting and air admittance valves are allowed. As it would be very hard to run a vent out of the house in this location my plan is to modify the plumbing to look like this:

enter image description here

My questions are these:

  • Am I good with code?

  • Do I need to have a second air admittance valve (AAV)?

  • Could I eliminate either the sink or shower vent?

  • Would this help alleviate the issues we had with the toilet backing up by getting rid of the 3x3x1.5 90 degree elbow under the toilet, or is there something else I am not considering?

  • Does the 'to main drain' connect to a vertical stack or is it actually just the drain to the sewer?
    – auujay
    Commented Aug 13, 2013 at 20:47
  • Forgive my incompetence with the plumbing terms as i try to explain how it works. The pipe with the arrow runs into the basement and connects to the main pipe that goes to the sewer. That pipe is vented through the roof... so i think the answer to your question is yes
    – Grimes
    Commented Aug 13, 2013 at 21:30
  • How are you planning to get the shower drain through all the joists? That's going to be a difficult run, and you need to verify you're not damaging the carrying capacity of the joists.
    – BMitch
    Commented Aug 14, 2013 at 11:08
  • That was the one good thing the previous owners had done, the joists are 2x10s. From what I've read 1/3 the depth of the joist will be fine for holes 2" away from the edge so i will have no problem fitting a 2" pipe through it. The 3" pipe runs parallel with the joists
    – Grimes
    Commented Aug 14, 2013 at 12:16
  • How far is the toilet from the main drain/vent stack?
    – HerrBag
    Commented Aug 14, 2013 at 12:44

1 Answer 1


Except as noted here, it all appears correct if I'm understanding your drawing correctly. The shower does need its own AAV, otherwise the resulting trap arm appears to be too long. Thus you should not eliminate either vent.

Changing the toilet venting will help immensely, provided it is done right. The 3x3x2 wye must be installed rolled up 45 degrees so that toilet waste cannot enter the vent line. Do not reduce the 2" line to 1.5", carry 2" up to a larger AAV.

  • Do i need to tilt the wye connection to the vent of the shower 45 degrees as well? I ask because I worry that i may run out of space between the 2x10 joists.
    – Grimes
    Commented Aug 14, 2013 at 12:21
  • Ideally yes. If the shower waste is run as low as possible and you roll the wye up as much as possible, I'd say it's adequate even if not quite 45 degrees. Any change in elevation is better than running the pipes level to each other, which I've seen too many times even though running the vent higher would have been simple. -sigh-
    – bcworkz
    Commented Aug 14, 2013 at 19:02

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